The use of transparent plastic materials as substrates for liquid crystal display devices instead of glass substrates is being studied. For example, the liquid crystal display device indicated in Japanese Unexamined Patent Publication No. 59-204545 has an electrode substrate comprising a 0.1-0.4 mm-thick thermoplastic resin [for example, PES (polyethersulfone), PMMA (polyethyl methacrylate), PC (polycarbonate) or PET (polyethylene terephthalate)]substrate, a 15 nm-thick SiO.sub.x undercoat film, a 20 nm-thick ITO transparent conductive film and an orientated directional film.
When the above-mentioned electrode substrate is introduced into the production process for an already existing liquid crystal display device in the same manner as a conventional glass substrate, the following problems occur.
1. In the alkali treatment and heat treatment processes following photoetching, cracks result in the SiO.sub.x undercoat film and ITO film, making further working impossible. Even if it is possible to fabricate a display device without such cracks, in reliability testing (high-temperature high-humidity preservation test, cooling/heating thermal shock test, etc.) cracks in the ITO film make, the ITO electrode broken, and cracks in the SiO.sub.x film generate air bubbles in the liquid crystal layer of the display section.
2. Since a thermoplastic resin is used, the rigidity (or "hardness") is insufficient even with a substrate with a thickness of 0.4 mm, and thus single-substrate-processing is difficult to perform in the same manner as for glass substrates. Furthermore, since the softening point of the substrate is about 100.degree. C., deformation occurs during the heating and pasting processes, and it becomes impossible to obtain uniform cell gaps, thus resulting in display irregularities.
3. In cases where the gas barrier properties of the SiO.sub.x undercoat are unsatisfactory, gases such as O.sub.2 and H.sub.2 O permeate the liquid crystal layer, resulting in the creation of air bubbles.